1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems use a network of base stations to provide wireless connectivity to one or more mobile units. In some cases, the mobile units may initiate wireless communication with one or more base stations in the network, e.g., when the user of the mobile unit would like to initiate a voice or data call. Alternatively, the network may initiate the wireless communication link with the mobile unit. For example, in conventional hierarchical wireless communications, a server transmits voice and/or data destined for a target mobile unit to a central element such as a Radio Network Controller (RNC). The RNC may then transmit paging messages to the target mobile unit via one or more base stations or node-Bs. The target mobile unit may establish a wireless link to one or more of the base stations in response to receiving the page from the wireless communication system. A radio resource management function within the RNC receives the voice and/or data and coordinates the radio and time resources used by the set of base stations to transmit the information to the target mobile unit. The radio resource management function can perform fine grain control to allocate and release resources for broadcast transmission over a set of base stations.
A conventional base station provides wireless connectivity within a geographical region that is referred to as a cell, a macrocell, and/or a sector. Conventional base stations can transmit signals using a predetermined amount of available transmission power, which in some cases is approximately 35 W for a base station. The range of the macrocell is determined by numerous factors including the available transmission power, angular distribution of the available power, obstructions within the macrocell, environmental conditions, and the like. For example, the range of a macrocell can vary from as little as 300 m in a densely populated urban environment to as much as 100 km in a sparsely populated rural environment. The coverage area can also vary in time if any of these parameters changes.
At least in part because of the reduced cost and complexity of deployment, simplified low cost base stations may be deployed in locations that are impractical for conventional base stations. For example, a low cost base station may be deployed in a residence or building to provide wireless connectivity to the occupants of the residents of the building. Base stations deployed in a residence are typically referred to as home base stations or femtocells because they are intended to provide wireless connectivity to a much smaller area (e.g., a femtocell) that encompasses a residence. Femtocells have a much smaller power output than conventional base stations that are used to provide coverage to macrocells. For example, a typical femtocell has a transmission power on the order of 10 mW. Consequently, the range of a typical femtocell is much smaller than the range of a macrocell. For example, a typical range of a femtocell is about 100 m. Clusters of femtocells may also be deployed to provide coverage to larger areas and/or to more users.
The functionality in a femtocell is typically quite similar to the functionality implemented in a conventional base station that is intended to provide wireless connectivity to a macro-cell that may cover an area of approximately a few square kilometers. A femtocell may therefore be deployed by a service provider as an integral and trusted part of a wireless network, in which case the femtocell basically operates as a base station with a relatively small range. However, femtocells may alternatively be designed to be inexpensive plug-and-play devices that can be purchased off-the-shelf and easily installed by a lay person. This type of femtocell, which is often referred to as a home femtocell, a private cell, or a home node-B, is not considered an integral or trusted part of the wireless network because it is not deployed or controlled by the service provider and is therefore vulnerable to hacking and other unauthorized uses.
Home femtocells are typically deployed as an overlay to the existing macrocellular network. The home femtocells may transmit in a different frequency channel or carrier to reduce interference with base station that support the macrocellular network. Each home femtocell may therefore transmit pilot(s) on its own serving carrier and transmit the beacon signals on all the collocated or neighboring macrocell carriers. User equipment can detect the presence of the home femtocell by searching for a beacon signal transmitted by the home femtocell in one of the carrier frequencies supported by the macrocellular network. The home femtocells can then provide wireless connectivity to registered user equipment when the user equipment comes within range of the home femtocell, detects the beacon signal, and hands off to the home femtocell. However, the macrocellular network, the home femtocell, and the user equipment can generate mutual interference. For example, when a macrocellular base station is providing power control instructions to the user equipment communicating with it, the user equipment may be instructed to increase its transmission power enough to cause interference with nearby home femtocells that operate on the same carrier. For another example, the beacon signal transmitted by the home femtocell can interfere with nearby user equipment that is using the same carrier to communicate with the macrocell. Each femtocell can also contribute to the intercell interference received by neighboring femtocells.
Conventional macrocellular systems are not configured to coordinate operation with conventional private femtocells. For example, neither conventional macrocellular systems nor conventional private femtocells gather location information to coordinate the operation. The macrocellular system and the private femtocell do not exchange user location or other user equipment information. Consequently, the private femtocell is not able to take any action it upon user equipment status in the macrocellular network. After a private femtocell is powered on, it will interfere with the overlaid macrocell and the neighboring private femtocells. Since the private femtocells are unaware of the user equipment status in the macrocellular network, the private femtocells can not be controlled adaptively to account for this variable status and more interference will be generated by the private femtocells. Moreover, handoff delay between the macrocellular network and the private femtocells may be increased and the power consumption of the user equipment and/or private femtocells may be increased. For example, when a private femtocell is powered on and begins transmitting its beacons and pilot(s), it will cause nearby user equipment to acquire the beacon/pilot and try to access the private femtocell. Since the most of the user equipment that are passing by are not the owner of the private femtocell, the repeated try and fail activities of the user equipment are wasting user equipment power. The unnecessary private femtocell transmissions also waste power.